We investigated the bark structure of woody liana Toxicodendron orientale Greene (Anacardiaceae) in ontogenesis and in the environmental factors of the gas-hydrothermal fields of Mendeleev volcano (Kunashir Island, Russian Far East). The following trends were found in liana ontogenesis: an increase in the diameter and length of the sieve-tube elements; sieve-tubes become rounded in cross section; an arrangement of clusters of the sieve-tube elements thereof contiguous with the axial and ray parenchyma and becomes more regular; and an increased volume of parenchyma in the conducting phloem. The age-specific transformations of the rest of the bark tissues are rather typical: dilation of the cortex parenchyma and the nonconducting phloem, sclerification of the parenchyma with isolated sclereid groups forming, obliteration of sieve-tubes etc. To study the structural changes of the cortex under environmental stress, we applied a comparative analysis of age trends to a large number of bark structural traits of the Toxicodendron orientale. Such age trends were created for traits of the bark in normal and stressful environments (sea coast and volcanic landscape). The structural changes of Toxicodendron orientale in response to extreme conditions near gas-hydrothermal springs are mainly related to the conducting phloem. Comparative analysis of the functional features of this tissue shows the following important trends for plants under extreme volcanic conditions: an inversion some of traits of the bark (bark thickness, secondary and conducting phloem width, total number of phloem rays, diameter and area of the sieve-tubes elements in the conducting phloem); a decrease of diameter and increase of length of the sieve-tubes elements in the conducting phloem; a diffuse arrangement of sieve-tubes in the conducting phloem in mature plants and also; a significant amount of the uniseriate rays.